I tried to google up something on this but didn't come up with anything too promising in the pages I scanned through so I'll share with you how I collected burdock seed and something along those lines may work for you. Collect your seed heads and let them dry out so they are nice and crunchy. Go outside - a must with all those little irritating fibers in burdock but it may not be as critical with thistle. Then again, who wants thistle thorns inside to step on? Find a sidewalk, a paved driveway, a garage floor, or some sort of concrete or stone surface. Put down something like a big paper grocery bag. Set some of the seed heads on it and crush with your shoes, walk on it, smush it, you get the idea. Obviously your not trying to turn the seeds into dust so take care with how much force you use. I haven't tried to collect thistle seed but as with burdock, this should break the seed heads up and free the seeds without pricking your hands to bits. In burdock, you don't want the nasty itchy fibers near skin. Anyways, you should be able to carefully pick out the big leftover plant bits. At this point with burdock you should be able to winnow or screen the seeds to rid the chaff. But thistles... I have found that in collecting seed from plants that have a fluff to the seed (ex. Joe Pye, Boneset, Valerian, etc.) that if I get the seed half processed by getting it out of the seed head and then let it sit and dry some more, the fluff gets more brittle and the seed may fall off by itself or may come free with some rubbing on a screen. Might work with thistle. I can not guaranty this will work but it is probably how I'd try it first..... FataMorgana

BANE! I've found this sort of "pretty" plant popping up all over my yard in metro-DC area. The really big plants are in my neighbors yard just out of my reach. I consider this plant to be BANE, because my mom had a similar issue with a neighbor that intentionally planted it. Pokeweed is EXTREMELY toxic. Both my mom's dog and her neighbor's dogs ended up in ER vet. Almost died. We think they ate the berries. My niece played with the berries and got very sick too. The purple bird poop is not fun either, but they spread the pokeweed if you lt it get to that point! I;m on a mission to get rid of it along with the poison ivy, sumac and oak I found hiding in my English ivy coming from roots in my neighbors yard. Don't get me started on what I learned today is kudzo climbing my Hemlocks and choking them out. I've got a toddler to look out for, and trying to keep these toxins away from the lovely little dog in my new neighbors yard that my daughter loves to talk to. I'm taking care of this organic way, just yanking, pulling and digging. I don't want to subject my kid to more toxins. Don't believe me? Check out http://extension.psu.edu/prepare/emergencyready/drought/dairylivestock/pokeweed among a million other sources that say it is bad! I joined gardenweb this evening because it was the first thing that popped up in my google search. I think I'm joining this discussion about 6 years too late for the original post, but thought some more balance needs to be brought to this discussion as the first url that came up on pokeweed.

Lavendrfem, Once the flowers fall off a pod of sorts (I'm assuming this is the mericarp) forms in its place on the stalk. It sort of looks like another flower is going to form. This is where the seeds are. You'll have to be patient, as it takes some time for that to form and then dry out. Once it's dried it does sort of start to open up on it's own. You can collect them while still on the stalk or cut the pods off and separate after. Each one of these pods must have about 25 - 40 seeds, very easy to collect. I have my first hollyhocks this year (nigra), grew them from seed and it took two years to get to flowering age. They are beatiful and produce so many seeds! Good luck collecting. Jenny

Thanks for the question and answer. I was pondering this same question this morning as I gazed out at my ever spreading canada thistle.